It is known that graphite is a high-thermal-conductivity material, but it is difficult to compacting only graphite. Thus proposed are graphite-particle-dispersed composites comprising such metals as copper, aluminum, etc. as binders. However, because graphite and metals do not have good wettability, there are too many boundaries of graphite particles in contact with each other when graphite particles exceed 50% by volume in the powder metallurgy method of producing composites from mixtures of graphite particles and metal powder, failing to obtain dense, high-thermal-conductivity composites.
To obtain dense, high-thermal-conductivity composites, attempts have been vigorously conducted to improve the wettability of graphite with metals. For instance, JP2002-59257 A discloses a composite material comprising gas-phase-grown carbon fibers having high thermal conductivity and a metal, the carbon fibers being coated with a silicon dioxide layer to have improved wettability to the metal. However, because carbon fibers are used, it suffers high production cost. And because a silicon dioxide layer having as low thermal conductivity as 10 W/mK is formed on the carbon fibers, the resultant composite fails to have sufficiently high thermal conductivity.
JP2001-339022 A discloses a method for producing a heat sink material comprising firing carbon or its allotrope (graphite, etc.) to form a porous sintered body, impregnating the porous sintered body with a metal, and cooling the resultant metal-impregnated, porous sintered body, the metal containing a low-melting-point metal (Te, Bi, Pb, Sn, etc.) for improving wettability in their boundaries, and a metal (Nb, Cr, Zr, Ti, etc.) for improving reactivity with carbon or its allotrope. However, it suffers high production cost because a porous sintered body of carbon or its allotrope is impregnated with a metal, and there is high thermal resistance between carbon or its allotrope and the metal because the low-melting-point metal and the reactivity-improving metal are added. Further, the impregnating metal has reduced thermal conductivity because it contains the low-melting-point metal and the reactivity-improving metal, failing to achieve high thermal conductivity.
JP2000-247758 A discloses a thermally conductive body comprising carbon fibers and at least one metal selected from the group consisting of copper, aluminum, silver and gold to have thermal conductivity of at least 300 W/mK, the carbon fibers being plated with nickel. However, it suffers high production cost because carbon fibers are used, and high thermal conductivity cannot be expected despite the use of carbon fibers because the carbon fibers are plated with Ni having low thermal conductivity.
JP10-298772 A discloses a method for producing a conductive member comprising the steps of depositing 25-40% by weight of copper on carbonaceous powder in a primary particle state by electroless plating, pressing the resultant copper-coated carbonaceous powder, and sintering it. However, this conductive member is used for applications needing low electric resistance and low friction resistance such as current-feeding brushes, and this reference has no descriptions about thermal conductivity at all. The measurement of the thermal conductivity of this conductive member has revealed that it is much lower than 150 W/mK. This appears to be due to the fact that because artificial graphite powder used has as small an average particle size as 2-3 μm, there are many boundaries between graphite powders, failing to efficiently utilize high thermal conductivity of graphite.